Browsing by Author "Arnold, MD"

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    Electrochemical energy storage on nanoporous copper sponge
    (Springer Nature, 2022-03-24) McPherson, DJ; Dowd, A; Arnold, MD; Gentle, A; Cortie, MB
    A proof-of-principle double-layer symmetrical supercapacitor with nanoporous copper/copper oxide electrodes and an aqueous electrolyte is investigated. The electrodes are manufactured by selective dissolution of Al from a eutectic composition of Cu17.5Al82.5 using 5 M NaOH. The ostensible (i.e., net external) capacitance of a symmetrical two-electrode cell with 0.1 M KNO3 electrolyte is assessed over a series of charge/discharge cycles and is about 2 F per gram of Cu in this simple prototype. Capacitance varies during a discharge cycle due evidently to the deeply buried surfaces and pseudocapacitive reactions contributing charge toward the end of a discharge cycle. In principle such a device should have very low ohmic losses due to its highly conductive backbone and would be suitable for applications requiring maximum energy efficiency over repeated cycling. The aqueous electrolyte ensures fire safety but this comes at the cost of lower energy content. © The Authors - Open Access under a Creative Commons Attribution 4.0 International License
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    Reconstructing the late Pleistocene climate sequence at Alexandra Cave, Naracoorte, using trapped charge dating and palaeoenvironmental proxies
    (American Geophysical Union (AGU), 2021-07-08) Priya, LJ; Arnold, MD; Guilarte, V; Cohen, TJ; Tyler, JJ; Demuro, M; Gadd, PS; Reed, EH
    The drivers of Australia-wide megafaunal extinction during the late Pleistocene remain poorly resolved. Hypotheses include individual or synergistic combinations of climate fluctuations, human impacts through hunting or habitat alteration by landscape burning. Moreover, the relationship between extinction dynamics and long term glacial - interglacial environmental change is not yet understood. Using a series of complementary geochronological, palaeoecological and geochemical techniques on a sedimentary sequence in Alexandra Cave, Naracoorte, this study provides improved reconstructions of past climates in south-east South Australia prior to, and around the time of, megafaunal extinction. Optically stimulated luminescence and electron spin resonance dating constrain the age of the sedimentary sequence to Marine Isotope Stage (MIS) 1 – 5. Preliminary environmental reconstructions undertaken using carbon isotopes of charcoal and Itrax micro X-ray fluorescence analysis reveal high precipitation during late MIS 5, while MIS 4 was arid. Decreased organic matter and increased sedimentation rates in mid-late MIS 3 suggest a change in landscape conditions, consistent with an environmental shift around the time of megafaunal demise locally. The Last Glacial maximum was arid, with frequent local fires, followed by a shift during the Holocene, marked by an increase in precipitation.
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    Spontaneous emergence of optically polarizing nanoscale structures by co-deposition of aluminum with refractory metals: implications for high-temperature polarizers
    (American Chemical Society, 2022-03-25) Tai, MC; Arnold, MD; Estherby, C; de Silva, KSB; Gentle, AR; Cortie, DL; Mitchell, DRG; Westerhausen, MT; Cortie, MB
    The unexpected growth of highly aligned and optically polarizing metallic fins during physical vapor deposition under modestly oblique conditions is investigated. The fins exhibit nanoscale dimensions and are formed when Al is co-sputtered with any of V, Cr, Nb, Mo, Ta, W, Ru, Fe, Ni, Pt, Zr, Mg, and Ti. It is proposed that the phenomenon is caused by anomalously low atomic mobility in the alloys and intermetallic compounds formed by co-depositing with Al. In contrast, when Cu, Ag, and Au (which diffuse more rapidly in Al) are deposited, no fins form. There is a sharp visible transition in optical properties as the ratio of Al to other element is decreased: the color of the sample changes from black to silver-white for compositions containing less than about 55 atom % Al. The region over which the color change occurs is associated with a very strongly polarized reflectance. Cross-sectional elemental mapping and Monte Carlo simulations suggest that growth of the fins may be nucleated by Al hillocks and enhanced by shadowing effects. The diversity of suitable metals makes this a versatile technique for producing nanoscale polarizing surfaces suitable for high-flux and high-temperature applications. © American Chemical Society